Dr. Pittaluga and I provide assistance in the diagnosis and classification of reactive and neoplastic lymphoproliferative disorders, immunodeficiency states, and diverse hematological malignancies. We provide consultative and collaborative services to physicians in the NCI, as well as to physicians studying patients with hematolymphoid disorders in other institutes, in particular NIAID, NHLBI, NHGRI, and NIAMSD. We regularly present at conferences sponsored by clinical branches in the NCI and other Institutes (NIAID, NHLBI, and NHGRI.) We receive more than 2000 cases in consultation each year. We try to restrict consultations to difficult or challenging cases, and do not accept cases for routine second opinions. A significant proportion (approximately 20%) of the cases are submitted by other academic institutions based on diagnostic uncertainty, or because of differences of opinion among several institutions. We are often the final arbiter on challenging diagnostic problems. Additionally, we have made novel observations based on our clinical practice, and a number of publications have emanated from case material originally reviewed in consultation. Our clinical consultation practice also synergizes with other NIH clinical groups, enhancing referral to NIH clinical protocols. With our international collaborators, a revision of the World Health Organization classification was published in 2017. It reflects a consensus among hematopathologists, geneticists, and clinicians regarding both updates to current entities as well as the addition of a limited number of new provisional entities. The revision clarifies the diagnosis and management of lesions at the very early stages of lymphomagenesis, refines the diagnostic criteria for some entities, details the expanding genetic/molecular landscape of numerous lymphoid neoplasms and their clinical correlates, and refers to investigations leading to more targeted therapeutic strategies. The revised classification reflects the most important advances in our understanding that impact our diagnostic approach, clinical expectations, and therapeutic strategies for the lymphoid neoplasms. In other work, our studies have led to improvement in the diagnosis and classification of several rare diseases including multicentric Castleman disease, Erdheim-Chester disease, autoimmune lymphoproliferative syndrome, KSHV- and EBV-associated proliferations, and rare histiocytic disorders, such as Rosai-Dorfman disease. The studies of Castleman disease involve large multi-center collaborative efforts, in which we play a major role, both in the design and conduct of the studies. As a member of an expert review panel, I review all cases submitted to a registry, to validate the diagnosis, and correlate with biological and translational studies. Another controversial and difficult area is the diagnosis and classification of lymphoproliferative disorders and lymphomas associated with diverse forms of immunodeficiency, including congenital, post-transplant, other iatrogenic causes, and acquired secondary to HIV, or advanced age with associated immune senescence. As one of eight members of an expert panel, I participated in an international workshop to evaluate the spectrum of disease and provide enhanced criteria for diagnosis and patient management. A perspective article has been submitted to Blood, summarizing the consensus and proposing new diagnostic criteria. I also serve on an expert review panel to validate the diagnosis of Burkitt lymphoma for a comprehensive TCGA analysis of this tumor.